In this dissertation, I examine the ability of free-ranging diurnal and nocturnal New World monkeys (black-chinned emperor tamarins, Saguinus imperator imperator, Weddell's saddleback tamarins, Saguinus fuscicollis weddelli; red titi monkeys, Callicebus cupreus cupreus; and Southern red-necked night monkeys, Aotus nigriceps ) to use visual cues, olfactory cues, spatial information, associative cues, landmark cues, and quantity information in making within-patch foraging decisions. This was accomplished through a controlled experimental field study conducted at the Zoobotanical Park of the Federal University of Acre (Rio Branco, Acre, Brazil) from August 1997 to July 1998. Analyses at the group level indicated that all four species were capable of learning the spatial distribution of food items within a patch and used this knowledge to return to previously exploited feeding sites. diurnal monkeys used visual cues to a greater extent than did night monkeys. Night monkeys, however, failed to show evidence of the use of olfactory cues more effectively than did emperor tamarins. In addition, when faced with conflicting spatial and perceptual information, all four species relied on visual or olfactory cues in selecting feeding sites. A comparison of the performances of emperor and saddleback tamarins when in and out of mixed-species association indicated that the dominant emperor tamarins may use information from the foraging behavior of saddlebacks to improve their searching efficiency. In contrast, saddlebacks performed better when out of association. Moreover, when forming mixed-species troops, both emperor and saddleback tamarins experienced costs of decreased time spent feeding and food intake. Analyses at the individual level indicated that among emperor and saddleback tamarins, group members may be classified as producers, scroungers, or opportunists, depending on their investment in searching for food. The adoption of these strategies is likely to be determined by factors such as social status and age. Differences in cognitive skills are unlikely to have an important role in the individual choice of foraging strategies. In addition, saddlebacks were more tolerant than emperors of sharing food rewards with other group members. Only adult male emperor tamarins, however, deferred to immatures and breeding adult females at feeding sites.